HIV infection in the brain causes severe neurological disorders in about 30 percent of AIDS patients. In the brain, HIV targets perivascular macrophages and microglia, however astrocytes and possibly endothelial cells are also infected. Persistent infection of astrocytes may represent a viral reservoir protected from current therapies that target actively replicating virus. The HIV envelope (env) interacts with CD4 and a 7-transmembrane coreceptor on cells to trigger virus entry. HIV-1 brain isolates use CCR5 as a coreceptor, however the possibility that HIV adapts for optimal replication in brain cells and/or exploits alternative coreceptors has not been fully investigated. For example, astrocyte infection in vitro occurs independently of CD4 and CCR5 via unknown receptors, and will resist new drugs targeted to CCR5. Most studies use viruses isolated in CCR5+, CXCR4+ blood cells. Such conditions will select against viruses adapted for brain coreceptors and brain cells. We propose to use established techniques to amplify complete env genes from brain tissue of HIV-1+ individuals. These envs, untainted by isolation methods, will be expressed on pseudotype reporter viruses to enable their biological properties to be analyzed. Specifically we aim to: 1. Investigate the extent HIV-1 variation in the immunoprivileged environment of the brain influences the biological phenotypes of envs present, including their sensitivity to new CCR5-specific compounds. 2. Analyze cell tropisms conferred by brain envs. High titer pseudotype viruses that carry brain derived envs will be used to test infection of primary blood macrophages and T-cells as well as brain microglia, astrocytes and endothelial cells. 3. Assess the extent alternative coreceptors are used by brain envs to infect primary cell cultures in aim (2). Data obtained will show if HIV env variation in brain effects interactions with CCR5 and establish the extent HIV adapts for alternative coreceptors on specialized brain cells. Our data will help assess if new therapies aimed at CCR5 will be effective in the brain and may reveal new coreceptors e.g. on astrocytes for therapy.